Method and apparatus for manufacturing tyres for vehicle wheels

ABSTRACT

In a method of manufacturing tires for vehicle wheels, a green tire, once formed, is transferred into a vulcanization mold and pressed against the holding walls of a molding cavity. Concurrently with the pressing step, a fluid present between the green tire and the holding walls is evacuated through vent valves. The tire is brought into contact with the closing head of each valve so as to push it toward a closed position at which the closing head forms a contact seat spaced apart from the inner surface of the molding cavity in order to define an embedding recess between the inner surface of the molding cavity and the closing head.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/IT2004/000729, filed Dec. 28, 2004, the content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and an apparatus formanufacturing tyres for vehicle wheels.

2. Description of the Related Art

Generally, in tyre manufacture for vehicles wheels, subsequently to astep of building a green tyre through assembling of respectivecomponents of elastomeric material some of which are provided withsuitable reinforcing structures, a moulding and curing treatment isprovided to be carried out to the aim of causing structuralstabilisation of the tyre through cross-linking of the elastomericcomponents of same as well as, as generally required, of impressing adesired tread pattern into the tyre itself, and possible distinctivegraphic marks at the tyre sidewalls.

To this aim, the green tyre is introduced into a suitably heatedvulcanisation mould, having a moulding cavity conforming in shape to thefinal conformation to be given to the tyre itself. After carrying outclosure of the mould, the green tyre is pressed against the holdingwalls of the moulding cavity and the heat required for tyrevulcanisation is simultaneously transmitted thereto. To reach thispurpose, for example, expansion of a bladder of toroidal conformation iscaused within the tyre through admission of steam under pressure intosaid bladder, so as to bring the bladder into contact with the innersurface of the tyre and compress the latter against the holding walls ofthe moulding cavity.

When vulcanisation has been completed, the mould is opened to enableremoval of the tyre and arrange the mould for a newmoulding/vulcanisation cycle.

Once the green tyre has been introduced into the mould, in order toprevent air pockets or pockets of other fluid possibly used in thevulcanisation process from being entrapped between the holding walls ofthe moulding cavity and the outer surface of the green tyre, the mouldsof the conventional type are provided with a plurality of venting ductspassing through the mould wall and opening into the moulding cavity,said ducts lying at right angles to the holding walls of said mouldingcavity. These ducts during the step of pressing the tyre against theholding walls of the moulding cavity, enable evacuation of possible airpockets and ensure a perfect adhesion of the outer surface of the greentyre to the holding walls of the moulding cavity. Adoption of ductspassing through and opening into the moulding cavity however, involves aplurality of problems due to seeping of the green-tyre blend into theducts themselves during the moulding step. In fact, the blend entrappedin the ducts is subjected to vulcanisation and forms a plurality ofthreadlike outgrowths on the finished tyre that are known in thespecific technical field as “flashes” or “burrs” and are generallyremoved in a work station dedicated to this purpose, when the tyreproduction process has been completed. Alternatively, these threadlikeoutgrowths are not removed from the outer surface of the finished tyreand in this case they involve worsening of the aesthetic-qualitativelevel of said tyre.

The state of the art proposes many solutions aiming at reducingformation of these threadlike outgrowths.

For example, in document CA-2,190,720 use of a movable valve is providedthat is inserted in each of the holes formed in the mould walls, saidvalve comprising a stem and a closing head associated with said stem.The closing head has a frustoconical shape provided with a flat surfacefacing the inside of the moulding cavity. A return spring pushes thevalve to the open position at which the closing head lies spaced apartfrom a respective seat of a mating shape, designed to receive said stemand closing head. The valve is forced to the closed position by thethrust exerted thereon by the tyre blend during the moulding step andcomes back to the open position when the tyre is removed from the mould.

Document GB-2,339,163 discloses a venting device comprising a closingelement slidably disposed within a main body of said device. A first anda second springs enable movement of the closing element—relative to themain body—so as to allow opening and closing of the device. At the innersurface of the moulding cavity, the main body of the device is providedwith a cone-shaped seat. At the upper end of the closing element thereis a closing surface of conical shape, matching the shape of thecone-shaped seat of the main body of the device. Closure of the valvetakes place when the cone-shaped seat and closing surface are engagedwith each other through movement of the closing element within the mainbody.

Document EP-1,380,397 discloses a venting apparatus positioned in a ventof a mould. Said apparatus comprises: a venting duct; a valve bodyinserted in said duct; a spring surrounding a lower portion of the valvebody to supply said valve body with an elastic force; and a retainer inengagement with a lower end portion of the valve body to controlvertical displacement of the valve itself. The inner diameter of theventing duct gradually increases towards the moulding cavity to define atapered shape. In addition, the valve body has a frustoconical closinghead provided with an inclined lower surface the inclination of which isdifferent from the inclination of the inner wall of the venting duct. Inthis manner, when the closing head comes into contact with the innersurface of the duct, contact does not take place along a surface butalong a circumference because in radial section the contact between thelower surface of the closing head and the inner surface of the duct isof the punctual type.

The Applicant has noticed that, in the open position, the lower taperedportion of the closing head of the known art valves (the above describedvalves, for example) faces the inside of the moulding cavity and greatlyprojects relative to the inner surface of said cavity, this surfacebeing defined by the holding walls of said moulding cavity. This aspectis particularly disadvantageous because in some regions of the mould, inparticular at the mould regions designed to form the tyre shoulders, theblend tends to flow tangential to the mould wall and to approach thevalve head in a lateral direction relative to said valves. In this waythe blend exerts a side thrust on the lower tapered portion of theclosing head, this thrust generating a force directed to move theclosing head away from the valve seat and to open the valve itselfinstead of closing it. Therefore, this force counteracts the thrustexerted by the blend approaching the closing head in a directionperpendicular to the inner wall of the moulding cavity and aimed atcarrying out the valve closure.

The Applicant has further noticed that the presence of blend interposedbetween the closing head and the seat of the valve compromises a correctclosure of the valve itself and brings to formation of faults on thefinished product. Repetition of this phenomenon during subsequentvulcanisation cycles further involves an increasingly greater storage ofcured blend within the valve, so that, cycle after cycle, an unavoidablestoppage of the valve itself will be caused.

Finally, the Applicant has ascertained that the side thrust of the blendon the closing head of the valve can cause misalignment of the valvestem relative to the duct in which the valve slides. Since a passagenecessary to enable the air to flow out of the moulding cavity isdefined between the stem and duct, the amount of the stem inclinationdue to the above described side force can be of such a value thatsliding of the valve within the valve seat and a correct air flow areimpaired.

SUMMARY OF THE INVENTION

The Applicant has therefore perceived the necessity to provide the mouldwith vent valves capable of preventing, or at least greatly reducing,blend seepage into the ducts of said valves and therefore avoid stoppageof same, as well as formation of the above mentioned threadlikeoutgrowths generally found in the known art embodiments.

The Applicant has found that the above mentioned problems can beeliminated or at least greatly reduced by carrying out coupling betweenthe valve seat and the head of the closure member of said valve in sucha manner that, at least in the closed position, the closure member headkeeps at least partly embedded in the mould wall.

In this way in fact in the open position of the valve, projection of theclosure member head towards the inside of the moulding cavity is greatlyreduced and therefore it is possible to greatly reduce blend seepages atthe inside of the valve duct even when the blend reaches said valvethrough its tangential movement along the holding walls of the mouldingcavity.

The Applicant has further found that the above problems can beadvantageously solved even when the closure member head keeps at leastpartly embedded in the mould wall also in the open position of the valveitself.

In a first aspect, the invention relates to a method of manufacturingtyres for vehicle wheels, comprising the steps of:

-   a) forming a green tyre;-   b) transferring the green tyre into a vulcanisation mould, the    holding walls of said mould defining an inner surface of a moulding    cavity, said inner surface conforming in shape to the final    conformation to be given to the green tyre;-   c) pushing the green tyre towards said holding walls of said mould;-   d) evacuating a fluid present between an outer surface of said green    tyre and the holding walls of said mould, said evacuating step    comprising the steps of:    -   d1) arranging at least one vent valve in said holding walls,        said vent valve comprising a duct having an inlet end opening        onto said inner surface of said moulding cavity and a closure        member having a closing head which is axially movable relative        to said duct; and    -   d2) transferring said fluid from the moulding cavity to the duct        through a connecting passage confined between an inner wall of        said duct and the closing head of the closure member set to an        open position;-   e) bringing the green tyre into contact with the closing head of the    closure member pushing the latter into the duct so as to achieve the    closed condition of said at least one vent valve;-   f) forming a contact seat in said inlet end to obstruct said    connecting passage, said contact seat being formed at a position    spaced apart from said inner surface of the moulding cavity so that    an embedding recess is defined between the inner surface of the    moulding cavity and the closing head of the closure member in the    closed position.

In a further aspect the invention relates to an apparatus formanufacturing tyres for vehicle wheels, comprising:

-   -   devices designed to form a green tyre;    -   at least one vulcanisation mould, the holding walls of said        mould defining an inner surface of a moulding cavity, said inner        surface conforming in shape to the final conformation to be        given to the green tyre, and    -   devices for transferring the green tyre into the moulding        cavity,    -   said vulcanisation mould comprising a plurality of vent valves        for said moulding cavity, at least one of said valves        comprising:        -   a duct having an inlet end opening onto said inner surface            of said moulding cavity, and        -   a closure member having a closing head that is axially            movable relative to the duct between:    -   a closed position, at which said closing head forms a contact        seat with an inner wall of said duct, said contact seat being        defined in said inlet end, and    -   an open position, at which said closing head and inner wall of        said duct confine a passage for connection with said moulding        cavity,        said contact seat being spaced apart from said inner surface of        the moulding cavity so that an embedding recess is defined        between the inner surface of the moulding cavity and the closing        head of the closure member in the closed position.

BRIEF DESCRIPTION OF THE DRAWINGS

A description will be set out hereinafter with reference to theaccompanying drawings given by way of non-limiting example, in which:

FIG. 1 diagrammatically shows in diametrical section, one half of avulcanisation mould being part of an apparatus for manufacturing tyresfor vehicle wheels in accordance with the present invention;

FIG. 2 is a partial section showing a vent valve according to a firstembodiment;

FIG. 3 shows an enlarged portion of the valve in FIG. 2;

FIG. 4 shows an enlarged portion of a vent valve made in accordance witha second embodiment; and

FIG. 5 shows an enlarged portion of a vent valve made in accordance witha further embodiment.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a vulcanisation mould belonging to anapparatus for manufacturing tyres for vehicle wheels in accordance withthe present invention has been generally denoted at 1.

This apparatus generally comprises devices adapted to form a green tyre2 and devices capable of transferring the green tyre 2 into a mouldingcavity 3 of the vulcanisation mould 1. Said tyre-forming andtransferring devices are neither shown nor further described as they canbe made in any convenient manner.

As shown in FIG. 1, the vulcanisation mould 1 has a pair of axiallyopposite cheeks 4 and a plurality of circumferential sectors 5 that,when the mould is closed, define the holding walls 6 of the mouldingcavity 3. The holding walls 6 delimit an inner surface 7 of the mouldingcavity 3 the shape of which matches the final conformation to be givento the tyre. The green tyre 2, once closed in mould 1, is pressedagainst the holding walls 6 by a suitable device 8. Subsequently, orsimultaneously with the pressing step, heat is administered to the greentyre 2 that is pressed against the holding walls 6.

By effect of pressing, suitable ridges set on sectors 5 and cheeks 4cause formation of a desired pattern on the tread band 2 a of the tyre,as well as of a plurality of distinctive graphic marks on the tyresidewalls. The administered heat causes cross-linking of the elastomericmaterial of which the tyre is made up. When a cycle has been completed,the finished tyre (i.e. moulded and cured) is extracted from thepreviously opened mould 1.

Shown in FIG. 1 by way of example is a pressing device 8 comprising abladder 9 of substantially toroidal conformation having twocircumferential edges carrying respective anchoring tailpieces 9 a to besealingly engaged in mould 1. A duct 10 for feeding steam or otherworking fluid and formed in mould 1 opens into bladder 9 so as to enableexpansion of the latter following admission of steam under pressure andto cause compression of the green tyre 2 against the cheeks 4 andsectors 5. Also operatively associated with mould 1, at the cheeks 4and/or sectors 5, are devices designed to supply heat to the green tyre2 to be cured, which preferably co-operate with the steam introducedinto the expandable bladder 9.

The vulcanisation mould 1 further comprises at least one vent valve 11for the moulding cavity 3 which performs the function of, concurrentlywith the pressing step, evacuating air pockets or pockets of other fluidpossibly used in the vulcanisation process, that are present between thegreen tyre 2 and holding walls 6.

As better shown in FIG. 1, the vulcanisation mould 1 comprises aplurality of vent valves 11, preferably mounted at the regions of mould1 close to the tyre shoulders and crown region.

Referring particularly to FIG. 2, each valve 11 comprises a duct 12having an inlet end 13 opening onto the inner surface 7 of the mouldingcavity 3 and an outlet end 14 opposite to the inlet end 13 and openingoutwards of mould 1, through suitable evacuation chambers 14 a (shown inFIG. 1), for example. For conveying the air out of the moulding cavity3, the evacuation chambers 14 a communicate with the externalenvironment of said cavity by means of channels not shown in FIG. 1.

Duct 12 is preferably defined along a valve body 15 inserted in anopening 16 formed in the holding wall 6. The valve body 15 is mountedflush with the inner surface 7 of the moulding cavity 3 in such a mannerthat an edge 15 a of the valve body 15 close to the inlet end 13 issubstantially coplanar with said inner surface 7.

Valve 11 further comprises a closure member 17 provided with a closinghead 18 axially movable relative to duct 12. The closing head 18 isrigidly connected to a stem 19 which is axially and slidably inserted induct 12. Stem 19 brings the closing head 18 close to a first end 19 apositioned in the vicinity of the inlet end 13 of duct 12. A second end19 b of stem 19, opposite to the first end 19 a and placed close to theoutlet end 14 of duct 12, carries a retainer 20 disposed externally ofthe duct itself. The function of retainer 20 is to prevent the closuremember 17 from slipping off the duct 12 towards the moulding cavity 3.Preferably, the retainer 20 is obtained through squashing of the secondend 19 b of stem 19, so as to form a swelling of said end that issufficient not to allow passage of the latter through the outlet end 14of duct 12.

The closure member 17 is movable between a closed position and an openposition. In the closed position (shown in FIG. 2 in solid line), theclosing head 18 forms a contact seat with an inner wall 21 of duct 12,said seat being defined at the inlet end 13, while retainer 20 keepsspaced apart from the outlet end 14. In the open position (representedin FIG. 2 in chain line), the closing head 18 and the inner wall 21 ofduct 12 are mutually spaced apart and confine a passage 23 forconnection with the moulding cavity 3. In this configuration, theretainer 20 lies in contact with the outlet end 14.

A helical spring 24 is disposed around stem 19 and is interposed betweenthe closing head 18 and an abutment surface 25 formed at the inside ofduct 12, preferably close to the outlet end 14. Spring 24 pushes theclosing head 18 to the open position, while the retainer 20 holds theclosure member 17 in duct 12.

Before the pressing step and during the starting moments of this step,the green tyre 2 does not fully adhere to the holding walls 6 of thevulcanisation mould land keeps at least partly separated therefrom.Under this situation, the vent valves 11 are maintained open by effectof springs 24. Compression of the green tyre during the moulding step(for example by means of the pressing device 8) and consequentapproaching of the green tyre 2 towards the holding walls 6 causesevacuation through the open valves 11, of the air (or other fluid)present between the holding walls 6 of the moulding cavity 3 and theouter surface of the green tyre 2. Said air is transferred from themoulding cavity 3 to duct 12 through the connecting passage 23 andtherefrom into the evacuation chambers 14 a. During the pressing stepthe green tyre 2 is brought into contact with the closing head 18 of thevent valve 11, pushing the closure member 17 into duct 12 and causingclosure of valve 11, against the action of spring 24, so as to obstructthe connecting passage 23.

Advantageously, the closing head 18 and the inlet end 13 of duct 12 aresuch shaped that, in the closed position (identified with a solid linein FIG. 3), the contact seat 22 is spaced apart from the inner surface 7of the moulding cavity 3, so that an embedding recess 26 is definedbetween the inner surface 7 and the closing head 18 of the closuremember 17. In other words, in the closed position, the closing head 18keeps at least partly embedded in the inner surface 7.

To this aim, in the preferred embodiments shown in FIGS. 2-5, theclosing head 18 has a peripheral edge 27 that, when valve 11 is in theclosed position, rests against the wall 21 of duct 12 so as to delimitthe contact seat 22. The contact seat 22 lies in a support plane 28 thatis offset relative to the inner surface 7 of the moulding cavity 3 by anembedding distance “d₁”. Preferably, this embedding distance “d₁” isincluded between 0.15 mm and 0.25 mm. As clearly shown in FIGS. 3, 4 and5, the bottom surface of the embedding recess 26 is made up of theclosing head 18 and a surface portion of the inner wall 21 of duct 12included between the closing head 18 and the inner surface 7 of themoulding cavity 3.

In order to obtain the embedding recess 26 when valve 11 is in theclosed position, the inlet end 13 of duct 12 diverges towards themoulding cavity 3 and opens onto the inner surface 7 at an opening 29the transverse sizes of which are greater than the peripheral edge 27 ofthe closing head 18. In the embodiments shown, the peripheral edge 27and opening 29 of duct 12 are of circular shape (in cross section) andthe diameter of the edge 27 is smaller than that of the opening 29.

Advantageously, the closing head 18 has a proximal portion 30 designedto engage into the inlet end 13 and on top of which there is a convexsurface 31 facing the moulding cavity 3.

The diverging inlet end 13 has a first angle α₁ defined by the summitangle of the cone tangent to the inner wall 21 of duct 12. The proximalportion 30 tapers towards stem 19 and has a second angle α₂ defined bythe summit angle of the cone tangent to the proximal portion itself. Thefirst and second angles α₁, α₂ have different widths from each other. Inparticular, at least at the contact seat 22 (i.e. in the closed positionof valve 11), the first angle α₁ is advantageously smaller than thesecond angle α₂ in such a manner that the contact seat 22 issubstantially defined (in the corresponding cross section) by acircumferential line. The mutual contact between the closing head 18 andthe contact seat 22 following a circumferential line enables thepossible occurrence of gluing phenomena to be avoided or at leastgreatly limited, between the closing head and inner wall of the duct,due to formation of important thicknesses of cured blend.

Preferably, at least at the contact seat 22, the first angle α₁ has avalue included between 55° and 65°.

Preferably, at least at the contact seat 22, the second angle α₂ has avalue included between 75° and 85°.

As pointed out in the embodiment shown in FIGS. 2 and 3, the inlet end13 of duct 12 has a frustoconical conformation (in the correspondingcross section). The proximal portion 30 too of the closing head 18 has afrustoconical conformation, the major base of which is delimited by theperipheral edge 27. Preferably, the summit angle α₁ of the cone to whichthe frustum of cone of the inlet end 13 belongs is smaller than thesummit angle α₂ of the cone to which the frustum of cone of the proximalportion 30 belongs. Preferably, the diameter of stem 19 is smaller thanthe diameter of the minor base of the proximal portion 30, and anannular surface on which an end of the helical spring 24 rests isdefined between stem 19 and said proximal portion 30.

Shown in FIG. 4 is a further embodiment, different from the one seen inFIG. 3. In detail, the alternative embodiment in FIG. 4 differs from thevalve in FIG. 3 because the inlet end 13 diverges according to acurvilinear profile and opens in a flaring shape towards the mouldingcavity 3. In this configuration, at the circumferential contact seat 22,the summit angle α₁ of the cone tangent to duct 12 is smaller than thesummit angle α₂ of the cone to which the frustum of cone of the proximalportion 30 belongs.

Shown in FIG. 5 is a further embodiment different from that shown inFIG. 4. In detail, the alternative embodiment in FIG. 5 shows the inletend 13 with a curvilinear profile as that in FIG. 4 and the closuremember 17 comprises an intermediate portion 33 extending in thecontinuation of the proximal portion 30. The intermediate portion 33tapers towards stem 19 following a third angle α₃ of a value smallerthan the second angle α₂. Preferably, the third angle α₃ has a valueincluded between 55° and 65°. Adoption of a summit angle α₃ of aparticularly reduced value enables displacement of the head 18 from theinner wall 21 of duct 12 to be limited, the lift of the closing head 18of valve 11 being the same, and as a result, enables the closure member17 to be guided within duct 12 in a more careful manner, so thatmisalignments of important amount of the closure member relative to theduct are avoided. In other words, the intermediate portion 33 disposedin the extension of the proximal portion 30 gives the closure member 17such a guide action that said member is not submitted to too markedinclinations and the smooth sliding of the latter in duct 12 isimproved.

Alternatively, according to further variant embodiments not shown, theproximal portion 30 of the closing head 18 and possibly also theintermediate portion 33, if present, can have a tapered configuration ofcurvilinear profile.

In addition, still within the scope of the present invention, it ispossible to provide coupling of a closing head 18 with double taper, asshown in FIG. 5, with an inlet end 13 having a frusto-conicalconformation, as shown in FIG. 3.

Alternatively, coupling of a closing head 18 of a curvilinear profilewith an inlet end 13 of a curvilinear profile can be provided, as shownin FIG. 4.

The convex surface 31 of the closing head 18 facing the moulding cavity3 is preferably of a frusto-conical conformation and tapers towards themoulding cavity 3, as shown in FIGS. 2-5.

Alternatively, the convex surface 31 of the closing head 18 has theshape of a spherical cap.

In the preferred embodiment shown in FIGS. 2-5, the frustum of cone ofthe convex surface 31 has a major base coupled with the major base ofthe proximal portion 30 of the closing head 18 and delimited by the sameperipheral edge 27 of the closing head. The minor base of the convexsurface 31 identifies a distal or top portion 34 lying spaced apart fromthe plane containing the peripheral edge 27 of the closing head 18 by apredetermined distance “d₂”. Preferably, this distance “d₂” is includedbetween 0.1 mm and 0.15 mm. Due to the inclination of the convex surface31 relative to the inner surface 7 of the moulding cavity 3, the sidethrust exerted by the blend moving along a direction tangential to themould surface (i.e. to the inner surface 7 of the moulding cavity 3)advantageously generates a component of force on the closing head 18which is directed to duct 12 and therefore tends to close valve 11.

In accordance with the illustrated features, in the closed position thedistal portion 34 of the convex surface 31 is housed in the embeddingrecess 26. The minor base of the frustum of cone delimiting the convexsurface 31 (i.e. the distal portion 34) is therefore offset relative tothe inner surface 7 of the moulding cavity 3 towards duct 12.

Alternatively, in the closed position, the distal portion 34 is flushwith the inner surface 7 of the moulding cavity 3 (this embodiment beingnot shown in the drawings).

As an alternative, in the closed position, the distal portion 34projects relative to the inner surface 7 of the moulding cavity 3towards the cavity itself, said projection being of a reduced amount(this embodiment being not shown in the drawings).

In both the above situations, the embedding recess 26 takes the shape ofan annular recess.

In accordance with the present invention, the lift of valve 11 is ofsuch a nature that, in the open position of the closure member 17, theperipheral edge 27 of the closing head 18 projects relative to the innersurface 7 of the moulding cavity 3 by a greatly reduced separationdistance “d₃” as compared with the solutions of the known art.Preferably, this separation distance “d₃” is smaller than 0.15 mm. Inthis position, the peripheral edge 27 of the closing head 18 is offsetrelative to the support plane 28 by a distance “d₄” greater than thedistance between the support plane 28 and the inner surface 7 of themoulding cavity 3, i.e. greater than the embedding distance “d₁”.

Alternatively and in accordance with an embodiment not shown, in theopen position of the closure member 17, the distance “d₄” between theperipheral edge 27 and the support plane 28 does not exceed theembedding distance “d₁”. In this case the peripheral edge 27 keepswithin duct 12 (and therefore in the embedding recess 26) even whenvalve 11 is in the open position.

As above described, coupling between the valve seat and the head of theclosure member of said valve is such obtained that, in the closedposition, the head of the closure member keeps at least partly embeddedin the mould wall. This expedient enables projection of the valve headtowards the inside of the moulding cavity to be greatly limited, thevalve lift being the same, when the valve is in the open position. Inthis way the invention, without impairing venting of the air from themoulding cavity, enables elimination of, or great reduction in, theamount of seepage of the blend reaching the head through a tangentialmovement relative to the surface of the mould. In this manner therefore,both the risk of stoppage of the valve and formation of surface faultson the finished products, such as the above mentioned thread-likeoutgrowths, is minimised.

In addition, the reduced projection of the closing head of the valverelative to the inner surface of the moulding cavity enables the thrustexerted by the blend tangentially reaching the closing head and tendingto move the head away from the valve seat to be eliminated or in anycase to be greatly limited.

Finally, the blend reaching the convex surface of the valve head bymoving tangentially of the inner surface of the moulding cavity producesa thrust component intended to close the valve. This thrust is to beadded to the thrust exerted by the blend approaching the closing head ina direction perpendicular to the inner wall of the moulding cavity andfacilitates timely closure of the vent valve, compensating for thetangential thrust exerted by the blend on the lower tapered portion ofthe closing head.

The invention claimed is:
 1. A method of manufacturing tyres for vehiclewheels, comprising the steps of: a) forming a green tyre; b)transferring the green tyre into a vulcanisation mould having holdingwalls defining an inner surface of a moulding cavity, said inner surfaceconforming in shape to the final conformation to be given to the greentyre; c) pushing the green tyre toward said holding walls of said mould;d) evacuating a fluid present between an outer surface of said greentyre and the holding walls of said mould, said evacuating stepcomprising the steps of: d1) arranging a vent valve in said holdingwalls, said vent valve comprising a duct, the duct having an inlet endopening onto said inner surface of said moulding cavity and an outletend opening outwards of the mould, and a closure member comprising aclosing head and a substantially cylindrical stem which are axiallymovable relative to said duct, the closing head comprising a proximalportion and an intermediate portion, the proximal portion comprising afirst truncated cone tapering toward the stem at a first angle andhaving a first upper diameter and a first lower diameter, the firstupper diameter being larger than the first lower diameter, theintermediate portion comprising a second truncated cone tapering towardthe stem at a second angle smaller than the first angle and having asecond upper diameter and a second lower diameter, the second upperdiameter being greater than the second lower diameter, wherein theintermediate portion extends in a continuation of the proximal portionand the first lower diameter of the proximal portion is substantiallyequal to the second upper diameter of the intermediate portion, andwherein the intermediate portion is adjacent the stem and the secondlower diameter of the intermediate portion is greater than a diameter ofthe stem, defining an annular abutment surface on a lower surface of theintermediate portion, the annular abutment surface abutting with aspring; d2) carrying out an open position of said vent valve, whilepreventing substantial misalignments of the closure member relative tothe duct by said intermediate portion at said open position; and d3)transferring said fluid from the moulding cavity to the duct through aconnecting passage confined between an inner wall of said duct and theclosing head of the closure member set to the open position; e) bringingthe green tyre into contact with the closing head of the closure member,pushing the latter into the duct so as to achieve a closed position ofsaid vent valve; f) forming a first contact seat in said inlet end bycontacting the proximal portion of the closing head to the inner wall ofthe duct in order to obstruct said connecting passage, said firstcontact seat being formed at a position spaced apart from said innersurface of the moulding cavity so that an embedding recess is definedbetween the inner surface of the moulding cavity and the closing head ofthe closure member in the closed position; and g) forming a secondcontact seat in said inlet end by contacting the intermediate portion ofthe closing head to the inner wall of the duct.
 2. The method as claimedin claim 1, wherein said evacuating step is conducted concurrently withsaid step of pushing the green tyre.
 3. The method as claimed in claim1, wherein said step of forming said first contact seat comprises thestep of arranging a peripheral edge of the proximal portion of theclosing head in a support plane that is offset relative to the innersurface of the moulding cavity.
 4. The method as claimed in claim 3,wherein said support plane is offset relative to the inner surface ofthe moulding cavity by an embedding distance of 0.15 mm to 0.25 mm. 5.The method as claimed in claim 3, wherein said step of carrying out theopen condition of said vent valve comprises the step of causing theperipheral edge of the proximal portion of the closing head to projectrelative to the inner surface of the moulding cavity by a separationdistance smaller than 0.15 mm.
 6. The method as claimed in claim 5,wherein in the open condition the peripheral edge of the proximalportion of the closing head is separated from the inner surface of themoulding cavity by a distance not exceeding the distance between saidsupport plane and the inner surface of the moulding cavity.
 7. Themethod as claimed in claim 1, comprising the step of arranging on theclosing head a convex surface facing the moulding cavity.
 8. The methodas claimed in claim 7, comprising the step of bringing a distal portionof the convex surface flush with the inner surface of the mouldingcavity.
 9. The method as claimed in claim 7, comprising the step ofcausing a distal portion of the convex surface to project relative tothe inner surface of the moulding cavity.
 10. The method as claimed inclaim 7, comprising the step of arranging a distal portion of the convexsurface internally of the embedding recess.
 11. The method as claimed inclaim 1, wherein in the closed position, the proximal portion of theclosing head acts on a circumferential line defining said first contactseat.
 12. The method as claimed in claim 1, comprising the further stepof administering heat to the green tyre pressed against the holdingwalls of the mould.